1. Field of the Disclosure
The present disclosure relates generally to the field of molecular biology, genetics and medicine. More particularly, it concerns compositions and methods for regulating the expression of GGGGCC-expansion disease genes, such as C9orf72, and thus the treatment of related diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).
2. Description of Related Art
An expanded hexanucleotide repeat has been implicated in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). This repeat expansion occurs in the first intron of the chromosome 9 open reading frame 72 (C9orf72) gene. It accounts for one-third of familial ALS and a quarter of familial FTD (Renton et al., 2011; DeJesus-Hernandez et al., 2012). The sequence of the repeat within C9orf72 pre-mRNA is GGGGCC. Patients with ALS or FTD typically have one mutant C9orf72 allele that contains 700-1600 repeats, while unaffected individuals have fewer than 24 repeats in both alleles (DeJesus-Hernandez et al., 2012). The C9orf72 locus also expresses an antisense transcript that encodes a CCCCGG repeat that may contribute to disease (Gendron, 2013).
Expanded repeats may form structures that disrupt normal RNA-protein interactions, affect RNA processing, and contribute to pathogenesis (Ling et al., 2013). The expanded sense and antisense RNA transcripts are C/G rich and the G-rich sense strand is known to form a stable G-quadruplex structure (Haeusler et al., 2014). The expanded repeats and the structures they form may sequester proteins and disrupt normal function by decreasing the effective concentrations of associated proteins within cells (Lee et al., 2013). A similar mechanism of action has been demonstrated for the expanded CUG repeats that occur with the mutant DM protein kinase gene responsible for myotonic dystrophy (Wheeler et al., 2009). Because of their potential to disrupt normal processes in cells and contribute to disease, both the sense and antisense repeat transcripts at the C9orf72 locus are targets for inhibitors that block RNA, disrupt structure, and alter the potential for RNA-protein interactions.
Both ALS and FTD are currently incurable, leading to an urgent need for new insights into treatment. One strategy to blunt the impact of mutant C9orf72 RNA is to inhibit expression of the gene. Antisense oligonucleotides that are complementary to intronic regions within the C9orf72 transcript have been tested. These oligonucleotides were designed to recruit RNase H to their target sites and lead to degradation of intronic RNA. Introduction of these oligonucleotides into cells caused foci formation to decrease, and reduced RNA toxicity (Donnelly et al., 2013; Lagier-Tourenne et al., 2013; Sareen et al., 2013). However, there are a number of hurdles in applying this kind of technology to the treatment of diseases like ALS and FTD that remain to be overcome.